I'm hyped for both games. When Horizon was shown last e3 it was exactly the kind of game I was waiting for to play. I was initially skeptical about Zelda after what they did with Skyward Sword but after watching the livestreams it looks like an amazing game.
Both games will be open world but they'll have different gameplay. Horizon wins in the graphics department and will look realistic and jaw-dropping but there's something about Zelda's gameplay that no other adventure series has topped so far in my opinion. I can't say which one I'm more hyped for.
I voted for Zelda because it looks more unique to me. Horizon seems to have more things in common with games like the Witcher so there's some sort of market saturation. But the last new 3D Zelda game released in 2011 (and that was a weak entry) so Zelda's got that little edge for me.